The amount of heat generated by a semiconductor device, for example, a silicon chip, is related to the number of transistors deployed on the device, the amount of power passed through the transistors, and the clock speed at which the semiconductor device is operated. Thus, as the number of transistors fabricated onto a single semiconductor device increases, the amount of heat generated by the device increases. Likewise, as the amount of power passed through the transistors increases, or as the speed at which the clock operating the semiconductor device increases, the amount of heat generated by the device also increases.
Since advances in semiconductor fabrication technology continue to yield ever more densely packaged high power transistors which are operated at faster and faster clock speeds, the problems associated with semiconductor device heat generation and dissipation are becoming more acute. In performance-driven applications, the highest clock speed options are generally preferred. However, since the highest clock speed options require more power and create more heat in a device, the difficulties associated with thermal buildup and dissipation are particularly prevalent.
One major problem associated with high power semiconductor devices is that as an increasing amount of heat is generated, the junction temperature of the transistors on the device increases proportionately. Those of skill in the art will appreciate that the failure rate of a semiconductor device is directly related to the junction temperature at which the device is operated. In other words, the higher the junction temperature, the greater the likelihood the device will fail. Accordingly, it is desirable to fabricate semiconductor devices in various package configurations which better facilitate heat transfer, and thus extend the life and reduce the failure rate of the devices.
In describing how generated heat is removed from a high power semiconductor device, it is convenient to consider the problem in two parts. First, there is a case-to-ambient thermal dissipation path, frequently called the external thermal path. The case-to-ambient path is dominated by a mechanism for transferring generated heat from the device directly out into the ambient environment.
Next, there is a junction-to-case thermal dissipation path, often called the internal thermal path. The junction-to-case path typically has a region directly in contact with the die in which heat flows predominantly in a vertical direction, i.e., in a direction normal to the active surface of the die, and then a region with a heat spreader in which lateral heat flow dominates. A heat spreader is any thick, thermally conductive plate used for laterally transferring heat on the package surface and for convectively removing heat away from an electrical device. Since the heat spreader diffuses heat laterally, the area through which heat can flow from the die to the ambient environment is effectively increased.
One problem with a conventional heat spreader is that the interface material used to attach the heat spreader to the back of the device creates additional thermal resistance in the heat transfer path between the die and the heat spreader. For example, a thick layer of epoxy is frequently used to attach the non-active surface of a semiconductor die to the bottom surface of a heat spreader. However, since epoxy has a very low thermal conductivity, it creates a large resistance for heat flow. Therefore, thermal dissipation capabilities for the package are reduced. Conversely, if the thermal resistance between the device surface and the heat spreader is reduced, the thermal performance of the package is improved due to the low resistance of the heat path.
Since high thermal resistance in the interface between a semiconductor die and a heat spreader can have a negative impact on both the performance and lifespan of a semiconductor package, it is therefore an object of the present invention to provide a high performance heat spreader which overcomes the above-mentioned difficulties by reducing thermal resistance and improving heat transfer processes. It is a further object of the invention to provide a heat spreader having a protrusion on the lower surface thereof which matches the size of a semiconductor die to which it is interfaced in such a manner that thermal resistance in the interface is reduced and better heat transfer characteristics are realized. Still further objects and advantages of the present invention will become apparent in view of the following disclosure.